Avena coleptile auxin is

Options

A. IBA
B. Indole 3-lactic acid
C. Indole 2-acetic acid
D. Indole 3-acetic acid.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about Avena coleoptile auxin, which refers to the type of auxin that affects the growth of the Avena (oat) coleoptile.

2. **Identifying Auxins**: The options provided are:
- IBA (Indole-3-butyric acid)
- Indole-3-lactic acid
- Indole-2-acetic acid
- Indole-3-acetic acid

3. **Understanding Bioassays**: A bioassay is a test that measures the biological activity of a substance, in this case, auxins, by observing their effects on plant growth. The Avena coleoptile test is a specific bioassay used to determine the activity of auxins.

4. **Recognizing the Avena Coleoptile Test**: The Avena coleoptile test specifically measures the effect of auxins on the curvature of the coleoptile. When an agar block containing auxin is placed on one side of the decapitated coleoptile, it causes the plant to bend towards the side without the auxin.

5. **Identifying the Correct Auxin**: The Avena coleoptile test is known to specifically measure the activity of Indole-3-acetic acid (IAA). This is the most common and biologically active form of auxin in plants.

6. **Conclusion**: Based on the information gathered, the correct answer to the question "Avena coleoptile auxin is" is **Indole-3-acetic acid (IAA)**, which corresponds to option D.

### Final Answer:
**Indole-3-acetic acid (IAA)**